The present invention relates to a method in which a plurality of tomographic images, for example, obtained by an X-ray computerized tomography (CT) apparatus or obtained by decomposing a volume image measured three-dimensionally by an MRI apparatus are stacked up to thereby obtain a stacked three-dimensional image (three-dimensional original image) and then two-dimensional images obtained by seeing the stacked three-dimensional image from arbitrary directions are shaded to construct a three-dimensional image (which means an image constituted by two-dimensionally arranged pixels but made to look like a three-dimensional image by shading).
Particularly, the present invention relates to a method and an apparatus, in which distortion created at the time of the projecting of the stacked three-dimensional image onto a two-dimensional plane is corrected and, further, the resulting image is displayed as if the inner wall of a tube-like tissue was observed under an endoscope.
Hereinafter, the "three-dimensional image" means an image projected onto a two-dimensional plane and shaded so as to look like a three-dimensional image. That is, the "three-dimensional image" is hereinafter distinguished from the stacked three-dimensional image (or three-dimensional original image).
In a conventional method of constructing a three-dimensional image, coordinate transformation according to parallel projection is used for transformation of coordinates of pixels into a coordinate system of a projection plane equivalent to a display screen.
In the parallel projection method used conventionally, a view point plane is not only a plane but also a projection plane. Accordingly, the parallel projection method is effective for constructing a three-dimensional image obtained by seeing a subject such as for example an internal organ, or the like, from the outside but is unsuitable for constructing a three-dimensional image obtained by seeing the subject from the inside, that is, unsuitable for constructing a three-dimensional image obtained by projecting a stacked three-dimensional image between a view point placed in the inside of the subject and a projection plane onto the projection plane.
There arises a problem that the parallel projection method cannot satisfy the recent demand that three-dimensional images should be obtained as if the inside of the subject was observed under an endoscope.
On the other hand, central projection method is one of projection methods used in the field of computer graphics. In the conventional central projection method, a point of view, a plane of projection and a subject of projection are arranged in order, so that the subject of projection is projected onto the projection plane while reduced in size. There arises a problem that resolving power is limited by a display matrix so as to be lowered compared with the case of 1:1 display.
The prior art concerned with projection methods has been described in the following literature.
"Fundamentals of Interactive Computer Graphics" by J. D. FOLEY & A. VAN DAM, translated by Atsumi IMAMIYA, pp. 277, 278, 297-302, issued on Jul. 15, 1984 by Japan Computer Association.